arm64: dts: freescale: use fixed index mmcN for NXP layerscape reference boards

The eSDHC driver has converted to use asynchronous probe.
Let's use fixed index mmcN for eSDHC controllers, so that
we can ignore the effect on usage, and avoid problem on
previous use cases with fixed index mmcblkN.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>